The present invention relates to a refrigeration circuit for circulating a refrigerant in a predetermined flow direction, comprising in flow direction a heat rejecting heat exchanger, an evaporator throttle valve, an evaporator, a compressor, an internal heat exchanger placed with its “cold side” between the evaporator and the compressor, a temperature sensor, and a control for controlling the evaporator throttle valve based on temperature sensor signals as provided by the temperature sensor.
Refrigeration circuits of this type the temperature sensor is located between the evaporator and the internal heat exchanger and they are operated in an operational mode which is called “semi flood”. “Semi flood” refers to the condition of the evaporator which instead of completely evaporating the refrigerant in the evaporator provides a mixture of gaseous and liquid refrigerant at its outlet which has a very low superheat. The internal heat exchanger will rise the superheat of this gaseous/liquid refrigerant, thus evaporating the remainder of the liquid refrigerant and securing safe operation of the compressor to which the refrigerant is directed subsequent to the internal heat exchanger. As is well-known, liquid refrigerant at the inlet of the compressor can cause severe damage of the compressor.
For optimum heat exchange in the evaporator the temperature sensor is provided at the outlet of the evaporator. Together with a measured pressure value, for example the suction pressure, the evaporating temperature and the superheat is calculated. Based on the temperature or superheat at the outlet of the evaporator the control controls the evaporator throttle valve and thus the flow of the refrigerant to the evaporator. Depending on the particular cooling requirement by the refrigeration consumer an optimum setpoint for the refrigerant flow through the evaporator can be maintained.
The system is, however, not dependent on the cooling requirement only, but also on other parameters like ambient temperature, etc. For example the condensation temperature rises up to 47° C. in summer time mode while it could be reduced down to 15° C. in winter time in order to optimise the energy consumption of the refrigerant circuit. This will result in a much lower capacity of the internal heat exchanger due to smaller temperature differences in the winter mode. In consequence liquid in the gaseous refrigerant may flow to the compressor because the capacity of the internal heat exchanger is too small. In summer mode, on the other side, the discharge temperature of the compressor could become critical, resulting in decomposition of the refrigerant and/or the lubricant as conventionally present in the refrigerant in some amount.